The present invention relates generally to medical devices and methods for deploying the same, and more particularly, to stents coated with a radiation-absorbing material and methods for inserting and expanding such stents within a lumen of the body.
Stents, including cardiovascular and biliary stents, are well known as devices that are used to support a body lumen, such as an artery, vein, biliary duct, or esophagus. They may be employed as a primary treatment for a constriction of a body lumen (stenosis), or may be used following a medical procedure, such as angioplasty, used to remedy stenosis. Percutaneous transluminal coronary angioplasty is one of the primary methods used to treat coronary artery disease. Percutaneous transluminal angioplasty of peripheral and visceral vessels as well as of other body lumens is also used to treat diseases known to be associated with those anatomical regions.
Conventional stents have taken two forms, each having a deployment method that is peculiar to the construction of the stent. First, there are self-expanding stents that typically are made of metal, and that may include a biocompatible coating. Generally, such stents are permanently implanted into the human body by deploying them on or through a catheter. The stent is placed in tension or compression at the distal end of the catheter, and percutaneously inserted into the body where it is guided to the site of implantation. The stent then is released from the distal end of the catheter, where it expands to a fixed, predetermined diameter, and is held in position as a result of that expansion and inward pressures exerted by the lumen.
A proposed variation on a self-expanding stent uses a material, such as nitinol, having a xe2x80x9cshape memory.xe2x80x9d A stent constructed from such material would be designed in a fully expanded configuration, then compressed into a second configuration so that it may deployed on a catheter to be inserted into the body percutaneously, and heated either before or after insertion into the body to return to its fully expanded configuration when released from the catheter. Alternatively, the compressed stent is chilled below body temperature, returning to its fully expanded state after the stent temperature has passively risen to body temperature following insertion into the body.
A second type of stent commonly used in the field is expandable as a result of mechanical action by the operator. One such stent is disclosed in Palmaz, U.S. Pat. Nos. 4,733,665, 4,776,337 and 4,739,762. According to the Palmaz patents, an unexpanded stent is permanently implanted in the body by percutaneously inserting it into a vessel using a catheter, and guiding the stent to the site where it is to be permanently implanted. Upon reaching the site of permanent implantation, the balloon portion of the catheter is inflated and the stent expanded, solely as a result of the mechanical force applied by the expanding balloon, until the stent is sized appropriately for the implantation site. Thereafter, the expanded balloon is deflated, and the catheter is removed from the body, leaving the stent permanently in position.
The use of thermo-mechanical techniques for forming polymeric structures within a body lumen is disclosed in Pathak et al., U.S. Pat. No. 5,741,323. According to that patent, a light-absorbing compound such as a chromophore is blended within a polymeric device that is intended to be deployed within the body lumen. The shape of this device may be modified in vivo by using a light source to heat the article to a temperature at which the material is flowable.
Because the chromophore is compounded into the polymer at a desired weight percentage, however, there is an increased risk of altering the stent""s mechanical properties, and thus its overall performance. Also, the sole purpose of the chromophore in the stent is to cause the stent to be heated for deployment; once this is accomplished, the chromophore is no longer needed in the body. Because the chromophore is compounded with the polymeric article throughout the entire polymer matrix, the chromophore persists in the body as long as the polymeric article remains in place. Thus, the risk of an adverse reaction to the chromophore is increased due to increased residence time and sustained release in the body.
Moreover, by blending the chromophore through the entire polymer matrix, selective heating of portions of the article is impossible, and indeed, because the interior of the article closest to the balloon will be heated first and longest, the method described by Pathak poses the risk that the balloon may be damaged by overheating. In addition, compounding the chromophore with the polymer by thermal means can alter its light-absorbing properties or degrade it significantly. Chromophores which are compounded with polymers and extruded to make stents may become photo-bleached, altering their absorption properties and making precise and repeatable deployment difficult. Chromophores can also chemically react with the polymer during heating, leading to cross-linking which may alter the physical properties of the polymer. Finally, as the polymeric stent expands, any fenestrations in the stent expand as well, creating regions devoid of chromophore and resulting in a drop in the efficiency of heating the stent.
Therefore, recognizing the desirability of expandable stents, it is necessary to provide a mechanism for deploying such stents within the body that overcomes the problems just noted.
The invention resides in a method for thermo-mechanically deploying a stent in a body lumen. The method comprises first the step of coating the stent with a radiation-absorbing material. The stent is placed at the distal end of a balloon catheter that includes either a radiation source at its distal end or a radiation source that is placed in the catheter lumen. The radiation source is selected to emit radiation that will be absorbed selectively by the radiation-absorbing material. The catheter thus rigged with the stent is inserted into the body lumen, and the stent is heated by generating radiation from the radiation source, which is absorbed by the radiation-absorbing material. Through this process, the radiation is converted to heat and used to warm the stent to a temperature above its glass-transition temperature to thereby become elastic, but below the temperature at which the stent becomes liquid or flowable. At this point, the heated stent is expanded by inflating the balloon catheter to a predetermined size. During expansion, stent temperature is maintained above its glass-transition temperature. Upon reaching that size, radiation is no longer supplied to the stent, which is allowed to cool below its glass transition temperature. The stent is thus no longer pliable, so that when the catheter is deflated, it may be withdrawn from the body leaving the expanded stent within the body lumen, the stent having sufficient hoop strength to support the lumen as desired.